Good musicianship is a synthesis of many musical elements, including musicality, performance expression, and proficiency in theory and sight-singing. To be a truly great artist, the singer has to combine the technical and emotional aspects of music. If you listen to your favorite singers, you'll probably find you love the sound of their voice, but also how they phrase the music and deliver the emotional content of the lyrics. These singers need an understanding of the music they're singing in order to accomplish this.